Ground track indicator



July 17, 1956 B. CHANCE ET AL 2,755,464

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ATTORNEY United States Patent GROUND TRACK INDICATOR Britton Chance, Cambridge, and Ivan A. Greenwood, Jr., Boston, Mass., assignors, by mesne assignments, to the United States of America as represented by the Secretary of War Application January 29, 1946, Serial No. 644,166

Claims. (Cl. 343-16) This invention relates to an airborne radar or radio object locating system and more particularly to apparatus in conjunction with the radar system for presenting on the indicator thereof data concerning the ground track of the aircraft. The ground track of the aircraft iis its path relative to the ground and -is therefore `an orthogonal projection 0n the ground of the aircrafts actual forward line of travel. Due to external forces such as wind, the ground track wil-l in general be different from the apparent direction of ltravel as indicated by the -aircrafts compass.

As disclosed in the copending application of Rulrby Sherr, enti-tled Method for Determining Drift Ang-le, Serial No. 624,907, tiled October 26, 1945, now abandoned, the ground track of an aircraft may lbe determined by `employing in the 'aircraft a radar or radio object locating system. In such a system echo pulses received at the aircraft from ground targets are inherently amplitude modulated due to the Doppler effect and lthe frequency of the amplitude modulation is a minimum when the center line of .the antenna of the radar system -is coincident with the ground track of the aircraft. The amplitude modulation of the returned echoes may be observed on the usual cathode ray tube indicator.

It is an object of the present invention therefore to provide electromechanical apparatus for pressing a continuous indication on 'a cathode ray tube indicator of the ground track of an aircraft.

I-t is a further object of the present invention to provide appara-tus which may be easily added to existing apparatus to provide the desired indication.

Other objects of the present invention will `become -a parent lfrom the 'following detailed description which is to `be read in connection with the accompanying drawings, Figure 1 of which illustrates in schematic block diagram form the preferred embodiment of the present invention. Figure 2 thereof shows a modified portion of the form of lFigure 1 illustrating an alternative form of the invention.

In 'Figure 1 of the accompanying drawing an antenna of a radar system, portions of which 'are not shown, is connected to a receiver 12 the output of which is connected to a cathode ray tube indica-tor 14, both o'f which also forma p a-rt of the radarl system. The radar system employed-is preferably -of the type disclosed in the cited copending application of Rubby Sherr andas such is capable of providing an indication by virtue of the. Doppler effect of the relative motion of the radar system and energy rellecting objects. The indicator 14 is preferably stabilized relative toV a xed direction such astrue formagnetic north by 4means of :a compass or gyro. 'The antenna 10 is'mech'anically connected to a motor 16,'preferably of the direct current type, and to the rotor lof a self-synchronous device 18. The rotor winding of the device -18 is energized by :an oscillator 20. ,Ihe statonwindings of the device '18 are connected to the'statr windings of a secondselfsynchronous device 22. The rotor of fthe device 22 is mechanically connected to a suitable handwheel 24. The rotor winding of the device 22 is electrically connected to voltage comprising amplifier 26 which is also connected to the oscillator 20. The ampliiier 26 may be any device well know-n in the art capable of comparing the voltage derived from the device 22 with @the reference voltage from the oscillator 20 to provide a D.C. output voltage which is indicative in polarity and magnitude of the error in angular position existing between the antenna 10 and ithe hand' wheel 24. Such a device will normally include a phase detector and suitable D.C. amplifiers or relays. The

D.C. output from the amplifier =26 is connected to two' terminals of a double-pole double-throw switch 28; The common terminals of the switch 28 are connected to the motor 16. The remaining two terminals of the switch 28 are connected to a suitable source of potential indicated by -l- `and signs.

A ground track mark generator 30 is connected to the indicator 1'4 and to the rotor winding of the `device 22. An alternative input to the ground track mark generator 30 is shown in Figure 2 of the drawing wherein the input to the ground track generator 30 is from the voltage comparing amplilier by means of the leads 32 rather than from the self synchronous device 22.

The portion of the apparatus in the drawing enclosed by the dashed b'lock 34 and which includes the motor 16, self-synchronous devices 18 and 22, amplifier 26 and oscillator 20 constitutes what is known in the art as a servo system. The term, servo system, refers to a mechanism, whereby one shaft, in Ithis case the lrotor Winding of the self-synchronous device 22 causes another shait, in this case the rotor winding of the self-synchronous device 18, to follow it in angular m-otion land it amplies the torque and power between the two.y The basic operation `of such a servo system is well known in the art .and its operation therefore will not be described in detail. To place the apparatus illustrated by the accompanying drawing in operation, the switch 28 is first placed in the position as shown connecting the output of the servo amplifier 26 to the motor 16. Rotation of the handwheel 24 will cause an error in the relative angular displacements of the antenna 10 and the handwheel 24 as measured from predetermined reference points with a resulting error in the displacements of the respective rot-ors of the devices 18 :and 22. This error 4in displacement will result in a voltage `output from the rot-or winding of the device 22. This error voltage is applied to 'the lamplifier 26 wherein it is compared with-reference signals from the oscillator 20 to provide a ID.C. output voltage which is applied through the switch 28 to the motor 16. The polarity of the D.C. output voltage is such that the motor 16 rotate-s the antenna 10 and the rotor of the device 18 in such a direction that the error in relaive angular displacement between the antenna 10 and liandwheel 24 is reduced to zero. The handwheel 24 is adjusted until the frequency of modulation of returned signal-s, as observed on the indicator 14, reaches a minimum value.` The :antenna 10 fis then pointing in the direction of the ground track of the aircraft and the positionY of the handwheel 24 is indicative of the angle betweenA the aircraft direction as indicated Vby its compass and the -ground track of the aircraft and may be calibrated to rread this difference if desired. The switch 28 is now placed in the opposite position such that the motor 16 is connected to the constant D.C. source. This will -result in the antenna 10 rotating continuously and in a presentation on the indicator -14 of the Plan Position Indication or PPI type in which all the targets Within the range of the radar system are portrayed .in polar coordif nate form. The handwheel 24 is not movedl from the position previously set.

There will be an error voltage from the rotor winding 3 of `the device'ZZ which varies in ,amplitude as the ant'a 71D 'Iiis 'tlrgh -Et "'plte "Y'Cl TljlS 'it voltage is `applied to the ground track mark kgenerator 30 Whiehtgenerates asuitable'voltlage which eausesillurnintion of one -or`more 4sweep traces lon the indicator T14. The` ground track mark generatorf'Sll may 'be-of 'the type disclosed and claimed in the fcopending application of Amasa 4S. Bishop entitled Electrical -"Circuit, 'Serial No. 598,157, tiled June 7, 1945, now Patent No. 2,547j3`63, Vissued pri13, 1951, or in -theappli'cation of France B. Berger and Frederick F. Slack entitled Electrical Apparatus, VSeri-al No. 611,658, tiled August 20, 11945, now Patent No. 2,572,975, issued'October'SO, `l51. "The circuits disclosed in the cited copendingapplications require that theinput 'thereto be` a modulated sinusoidal voltage andtheinput `thereto must `'therefore be derived from the device 22. The -ground track mark-generator A30 generates a voltage a-t the instant when'the output'from -the device yZ2 passes through Zero. It will be Obvious that `this condition corresponds to the conditions present when the initial adjustment of the handwheel 24 was made jand therefore 'that theindicationpropei'ly represen-ts "the ground track of the aircraft. Othercircuits for generating the desired voltage for application tothelindi- Vcator 14 may require a DQC. Voltage with changes in polarityat 'the proper time, and for this reason Ithe alternate connection to the `amplifier 26 indicated by the lines 32 is shown.

While there has been described what is at present considered'the `preferred embodiments of E'the present invention, it will be obviousto those skilled lin the antlthat various changes :and modifications may be made therein without departing from ythe scope of the invention. lFor example, one obvious modication would be the substitution of other data transmitting devices such aspoten-y tioxneters instead of 'the self-synchronous devices herein described. f

W'hat is `claimed is:

1. r'In a radio object locating `system including a directive antenna, driving means coupled thereto-for rotating said antenna, a power supply'for operating said driving means, and a signal indicator coupled to said antenna, the apparatus for producing an indie-ation 'of a pre`determined instantaneous position of said antenna as it rotates comprising an adjustable angular position control means, f

anA electromechanical device coupled between said control means 'and saidantenna for-generating a differential voltage indicative of the relative 'angular displacement of said antenna and said control means, `switching meansin one position coupling the output of said electromechanical device v'to said driving means for causing said antennato take a predetermined position in accordance with the position of said control means, said switching meansuin another position coupling said power supply to said driving means for rota-ting said antenna, and means coupled between said electromechanical device and said indicator for generating in response to said ditferentialvoltage a signal indicative of the instants that the antennaisin said predetermined position and for applying saidsignal to said indicator for obtaining an indication of saidpredetermined position as `said rantenna rotates..

2. `In a radio object locating system in amairplane including a directive receiving antenna, driving means'coupled thereto for rotating said antenna, arpowerlsupply for'energizing said driving means, and va signal indicator coupled to said antenna, the apparatus for producing a continuous indication of the actual course ofthe-aircraft w'itfhrespect to ground as the antenna rotates comprising an adjustable rotary position control means 'for said an- At'e'rr1a,'an electromechanical device Vcoupled between said control means and said antenna for'generafting Aa ldifferential 'voltage indicative of the relative angular displacement Y`ol'said antenna and said control means, -a voltagefra'nsforming device coupled to said electromechanical voltage a 'direct voltage AVrepresentative Ain magnitude "and polarity of the relative angular displacement yof said antenna and said control means, switching means in one position coupling the output of said voltage transforming device to said driving means for causing said antenna to take a predetermined angular'position in accordance wit-h the position of said control means, said predetermined position being along the actual course of said airplane, said switching means in another position connectingsaid power supply to said driving means for 'continuously rotating said antenna, and means responsive to said differential voltage for generating la signal indicative f the instants that theantenna is directed 'alongfsaid actual course and for applying said ysignal to said indicator for obtaining a continuous indica-tion of said actual course as said antenna rotates.

3. Apparatus according to claim 2 in which said lastnientioned generating means is coupled between vsaid voltage transforming device and lsaid indicator for generatingy said signal inresponse'tolsaid 'dire'ctvoltage 4. kItrppaiatus according toclaim 2`inwhich saidlastmentioned generating means is coupledbetween' said'electrcmeclranical device :and said indicator for generating said signal directly inresponseto said difieren-tial voltage.

'5. yIn a 1radio -object locating system in an `airplane including a directive "antenna, `driving means 'mechanically connected to said antenna for'r'oba'ting the same, a power supply for continuously operating said driving means, and a signal indicator coupled to said "antenna, the apparatus for producing `a continuous'indica'tionof the actual 'course of the 'aircraft withirspect 'to 'ground as the antenna `rotates comprising tirstseltl'synchronous means including a'stator and a ro'tormechanically connected to said antenna, 4an adjustable angular position control'mea'ns, second self-synchronous'means including as'tatorand a rotor mechanically'connected@to safd'con tr'oltneans, signal generating means 'electrically connected to'tlie'rotor of said first'self-synchronousmerma 'means electrically interconnecting the stators ot'bothsaidselt synchronous means, voltagecomparing'means Connected to the rotors or botlrsaid self-synchronous means 'for producing 'a differential voltage'indi'cative in magnitude andpolarity of the relative-displacement of' said antenna and sind control means;swi'tchingmeansin 'one position connectingkthe output of said voltage Yconniar'i'n'gmeans to said drivingmeans for causing said antenna to take a predetermined position in accordancewiththe'position of said control means and in 'anotherpo'sifion connecting said power supply te said driving means Yfor continuously rotatingsaid antenna, said predeterminedposition'beingalong thea'ctual course ofsaid airplanefand me'ans coupled bctween the rotor of said `second'seit-synclironous 'means andsaifd indicator' for generating i`n"response"to 'the signal iu'duced lin said rotor marker pulses :indicative of the instance th'atthe 'antenna is directed "along said 'actual course and for applying said marker pulsesto'said indicatorfor producing acon'tinuous indication of said actual cou'rse'as' said Aantenna rotates` References Cited in the'jle of4 this patent j UNITED V'STATES PATENTS s v kur 

